Data Refresh Fails, Error message claims duplicate ID value in table

Hi, I was having this issue as well.  In my case, the Query is doing a 'groupby' on order numbers , and capturing the minimum dates associated with the order number.    It would refresh on desktop, but gave the duplicate value error when refreshing on the server.    By the way, it was refreshing fine on the server for months, but the error started recently.   Further investigation into the raw data, I found a few cases when the order numbers had a trailing space that shouldn't be there.   So it seems the group by function will result in 2 different rows/orders ( with and without the space), but when you look at the resulting groupby results , the trailing space is not there, so the uniqueness is lost.    Adding a 'trim' function to my query to eliminate the trailing spaces on my order numbers, before the Groupby step,  resolved my issue.   Hope this helps someone.  cheers